Don't worry about cross chaining unless you discover that your bike has lots of trouble with it. Loud noises or dropping the chain will tell you it's having trouble. Otherwise, just pick a gear that's easy to pedal at a moderate cadence. Work on keeping your cadence up, and don't feel bad if at first you get tired out quickly. It takes time to get stronger.

If it's a new bike, it probably has indexed shifting. This is good, because the gears are numbered. Big number means harder gear. Small number, easier gear. So 3/8 would be your hardest gear, and 1/1 would be your easiest one. My bike is set up so shifting feels a lot like using the brakes. The easiest way to get comfortable with shifting is to start out on the flat and just practice.

Pick one chainring (the big gears in front) to work on, and play around with all the different gears in back. Then you *know* which gears are easy and which are hard for you. On some other ride, pick a different chainring. After you've done all the chainrings, you should have a list of good flat gears. Combining this with downshifting for stop signs should get you a lot more comfy with the shifters and balancing and brakes. Then hills shouldn't feel so scary . And if you start to feel unstable, stop and walk the bike. There are lots of times where it's not smart to ride through.

The other good part of doing this is you can see which chainring has the most lower gears available to you for hills. You also have an easy source of fitness goals. If the 3rd chainring is just plain too hard, you can make being able to use it a goal. When I got my bike in May, I couldn't ride on the 3rd chainring at all. Now I can sometimes cruise along in 3/6 for a hundred yards or so, and I'm slowly re-learning my hill climbing skills.